The invention relates to a method of determining a nuclear magnetization distribution in a region of a body which is situated in a steady, uniform magnetic field. In the method, spin resonance signals are generated in the region of the body in first and second sequences by high-frequency excitation pulses and are measured in the presence of magnetic field gradients superposed on the uniform magnetic field. The excitation pulse in one sequence is preceded by a high-frequency inversion pulse which inverts the nuclear magnetization. Using a transformation, a first image of complex image values is formed from said first sequence and a second image of complex image values is formed from said second sequence, each pixel in the first image and the second image being represented by an image amplitude and an image phase.
The invention also relates to a device for determining a nuclear magnetization distribution in a region of a body, including:
means for generating a steady, uniform magnetic field, PA0 means for generating high-frequency excitation pulses, PA0 means for generating magnetic field gradients which are superposed on the uniform magnetic field, PA0 means for measuring spin resonance signals, processing means for processing the measured spin resonance signals, PA0 control means for controlling the above means, PA0 said control means being suitable for the measurement of different sequences, PA0 said processing means being suitable for combining measurement results from two different sequences and for determining images from the sequences.
Such a method and device are known from EP No. 0.145.276.
Devices for determining a nuclear magnetization distribution in a region of a body and the principles on which the operation of such devices is based are known, for example from the article "Proton NMR tomography" in Philips Technical Review, Volume 41, 1983/84, No. 3, pages 73-88. Reference is made to this article for the description of their construction and principles.
In a method described in EP No. 0.145.276, a body to be examined is exposed to a steady, uniform magnetic field Bo during a so-called saturation recovery measurement wherein a selective 90.degree. excitation pulse is followed by a non-selective 180.degree. pulse in order to obtain an echo signal. The steady, uniform magnetic field Bo has a direction which coincides with, for example the z-axis of a cartesian coordinate system (x,y,z). The steady magnetic field Bo causes a slight polarization of the spins present in the body and enables spins to perform a precessional motion about the direction of the magnetic field Bo. After the establishment of the magnetic field Bo, a magnetic field gradient which acts as a selection gradient is applied and at the same time a 90.degree. rf pulse is generated which rotates the magnetization of the nuclei present in a selected slice through an angle of 90.degree.. After termination of the 90.degree. pulse, the spins will perform a precessional motion about the field direction of the magnetic field Bo, thus generating a resonance signal (FID signal). After the 90.degree. pulse, there are simultaneously applied field gradients G.sub.z, G.sub.x and G.sub.y whose field direction coincides with that of the magnetic field Bo and whose gradient directions extend in the z, the x, and the y direction, respectively. The field gradient G.sub.z then opposes the G.sub.z applied during the 90.degree. pulse and serves for rephasing the excited spins in a slice perpendicular to the z direction. The field gradients G.sub.x and G.sub.y serve for encoding in the x direction and the y direction, respectively. After termination of the three noted field gradients and after application of a 180.degree. echo pulse, a field gradient G.sub.x is applied and an echo resonance signal of the original FID signal is sampled, after which an image is reconstructed after Fourier transformation. In the cited European Specification EO No. 0.145.276 it is noted that the phase of the pixels generated by means of this so-called saturation recovery measurement can be disturbed by inter alia eddy currents which arise in conductive parts of an MRI apparatus due to the application or termination of magnetic field gradients during a measurement. In order to eliminate, or at least mitigate, phase disturbances in pixels generated by means of a saturation recovery measurement, the cited Patent Specification EP No. 0.145.276 proposes an extension of the method in the form of an inversion recovery measurement, including a non-selective 180.degree. pulse in order to obtain an echo signal. The pulse sequence thereof has the same appearance as that of a saturation recovery measurement, be it that for an inversion recovery measurement the pulse sequence is increased to include a high-frequency electromagnetic 180.degree. pulse which is generated some time before the high-frequency electromagnetic 90.degree. pulse. It will be apparent that the time sequence in which the saturation recovery measurement and an inversion recovery measurement are performed is irrelevant. By determining the quotient per pixel of the images obtained from an inversion recovery measurement and a saturation recovery measurement, respectively, ideally an image without phase errors will be obtained, provided that the two types of measurement have both been performed under the same experimental conditions so that the phase errors cancel one another.
A drawback of the known, extended method is that the quotient of the cited Patent Specification EP No. 0.145.276 contains an imaginary term which represents the difference between the phase errors in the two sequences and which is not always negligibly small in practice, so that corrected pixels will still contain phase errors which give rise to annoying differences in intensity in a corrected image. A further drawback is that the inversion recovery measurement does not produce a pure inversion recovery image, but rather, in conjunction with the saturation recovery measurement, a proportional image.